This paper presents an experimental study of the performance and emission characteristics of a port fuel injection spark ignition engine modified to fuel by compressed natural gas (CNG). Consequently, the original fuel supply system of the test engine was modified to flexibly run on either gasoline or CNG. The experiment was conducted to compare operating parameters of the test engine fueled by gasoline and CNG with the same air excess ratio (λ). The study results showed that when fueled by CNG, the brake power of the test engine decreases up to 19% and the brake specific energy consumption (BSEC) increases 14%. However, the exhaust emissions of the test engine fueled by CNG are enhanced dramatically. Indeed, CO2 emissions decrease up to 50%, NOx emissions decrease 20%, especially, CO and HC emissions reduce up to 90% and 96%, respectively. For the application of CNG in real-world conditions for in used vehicles, it is recommended that we have to concentrate to solve the problem related to the low power and high fuel consumption of the original engines fueled by CNG.